The present invention relates, in general, to a method and device for maintaining a position of a load which may or may not be secured to a load-carrying member, e.g. spreader. In the following description, a reference to “load” generally implies a reference to the load itself or also to the load-carrying member which carries the load.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
When swing of a load takes place, as the load is lifted by a lifting gear, such as a crane, the load may be caused to go out of plumb. In particular, in the case of a crane, a precise and safe cargo transfer is however desired for cost reasons. Therefore, the occurrence of a swinging motion as a result of external interferences such as, e.g., wind or uneven loading of a container, should be compensated.
In the following description, the term “swinging motion” or “swing” will be used in a generic sense and is used synonymously and interchangeably with rotation, wobble and/or skew motion or slewing motion. Swinging motion may also involve a movement about a rotation axis whereby the load is deflected at least once or several times from the rest position to an inversion point before returning to the rest position.
German publication no. DE 100 06 486 A1 describes a crane having a trolley and two drum-type hoisting mechanisms, each having two cable drums and a drive unit. Swing of the load can be compensated only in the travel direction of the trolley.
German publication no. DE 20 535 90 describes a lifting gear for loading containers. The lifting gear has a gripper frame and a hoisting mechanism as well as a control unit and a measuring device. The hoisting mechanism includes three lifting units, whereby each lifting unit has a cable drum and a separate drive. While slanted dispositions of the gripper frame can be eliminated before a container is deposited, a swing of the load cannot be compensated.
Heretofore, conventional lifting gears are unable to counter a swinging motion of a load in relation to an axis of the load. Thus, the prior art fails to address the problem of accurate positioning of a load, especially of maintaining a horizontal disposition of a load. The term “positioning” relates in this context to static loads as well as dynamic loads, i.e. masses undergoing changes in motion.
It would therefore be desirable and advantageous to provide an improved method and device for maintaining a position of a load to obviate prior art shortcomings and to address the problem of load swing, in particular about an axis in lifting direction, so as to realize a precise, quick and safe cargo transfer.